Prognosis of Myocarditis Developing After mRNA COVID-19 Vaccination Compared With Viral Myocarditis

BACKGROUND: Association between messenger RNA (mRNA) COVID-19 vaccines and myocarditis has aroused public concern over vaccine safety. OBJECTIVES: The goal of this study was to compare the prognosis of this condition with viral infection–related myocarditis over 180 days. METHODS: A territory-wide electronic public health care database in Hong Kong linked with population-based vaccination records was used to conduct a retrospective cohort study. Since the roll-out of BNT162b2 (Pfizer-BioNTech), patients aged ≥12 years hospitalized with myocarditis within 28 days after BNT162b2 vaccination were compared against viral infection–related myocarditis recorded before the pandemic (2000-2019), over a 180-day follow-up period (starting from diagnosis of myocarditis). All-cause mortality, heart failure, dilated cardiomyopathy, heart transplant, and postdischarge health care utilization were examined with Cox proportional hazards models. RESULTS: A total of 866 patients were included for analysis. Over the follow-up period, 1 death (1.0%) of 104 patients with postvaccination myocarditis and 84 deaths (11.0%) of 762 patients with viral infection–related myocarditis were identified. One case (1.0%) of dilated cardiomyopathy and 2 cases (1.9%) of heart failure were identified in the postvaccination group, compared with 28 (3.7%) and 93 (12.2%) in the viral infection–related myocarditis group, respectively. Adjusted analysis showed that the postvaccination myocarditis group had a 92% lower mortality risk (adjusted HR: 0.08; 95% CI: 0.01-0.57). No significant differences in other prognostic outcomes were seen. CONCLUSIONS: This study found a significantly lower rate of mortality among individuals with myocarditis after mRNA vaccination compared with those with viral infection–related myocarditis. Prognosis of this iatrogenic condition may be less severe than naturally acquired viral infection–related myocarditis.